Synthesis and characterisation of transition-metal complexes involving cyclic diselenoether ligands

Abstract

Treatment of NCSe(CH₂)₃SeCN with o-C₆H₄(CH₂Br)₂ in Na–NH₃–tetrahydrofuran yielded L² (3H-1,4,5,7-tetrahydro-2,6-benzodiselenonine) in greater than 80% yield. This cyclic diselenoether has been fully characterised spectroscopically and structurally. Reaction of [Cu(MeCN)₄]Y (Y = BF₄ or PF₆) or AgBF₄ with 2 molar equivalents of L¹ (1,5-diselenacyclooctane) or L² afforded the mononuclear species [M(L)₂]Y (M = Cu, Y = BF₄ or PF₆; M = Ag, Y = BF₄). These compounds have been characterised by IR and ¹H NMR spectroscopy, mass spectrometry and microanalysis. Crystal structure determinations on [Cu(L²)₂]BF₄, [Ag(L¹)₂]BF₄ and [Ag(L²)₂]BF₄ confirmed the discrete molecular nature of these products, giving distorted tetrahedral cations in each case. These structures are compared to the structures of related species involving acyclic bidentate thio-, seleno- and telluro-ethers. The preparation of the tetraselenoether macrocyclic complex [Ag([16]aneSe₄)]BF₄ ([16]aneSe₄ = 1,5,9,13-tetraselenacyclohexadecane) has also been determined. Compounds L¹ and L² reacted with 2 molar equivalents of [AuCl(tht)] (tht = tetrahydrothiophene) to yield [(AuCl)₂L¹] or [(AuCl)₂L²] respectively as white solids. These neutral compounds are thought to involve linear Se–Au–Cl fragments. Compound L² reacted with 1 molar equivalent of MCl₂ (M = Pd or Pt) in refluxing MeCN to give cis-[MCl₂(L²)]. Unusual low-frequency co-ordination shifts [δ(free ligand) – δ(complex)] were revealed by ⁷⁷Se-{¹H} NMR spectroscopy in each case. This is in contrast to cis-[MCl₂(L¹)] which display high-frequency co-ordination shifts. The ¹⁹⁵Pt NMR spectroscopic data on cis-[PtCl₂(L²)] are consistent with an Se₂Cl₂ donor set at Pt^II. The crystal structure of cis-[PdCl₂(L²)] shows a distorted square-planar arrangement, with d(Pd–Se) slightly longer than in cis-[PdCl₂(L¹)].